Bitrix24 mobile app does not provide a Mail option inside the Bitrix24 app, because in Bitrix24 web interface Mail section is a connection between Bitrix24 & Mail client (gmail, Bitrix24 own name (provided by Yandex, etc.) , which means the Mail option in the mobile app should redirect you to the mail client, which can be diffrent for different Bitrix24 users - gmail, yandex (for @bitrix24.com, or other)For @bitrix24.com own email addresses you can install Yandex.Mail app at your phone & log in with your @bitrix24.com email address.